Fluoride is a biologically relevant anion. Insufficient dietary intake of fluoride results in poor dental health, osteosclerosis, and osteoporosis. On the other hand, excess fluoride intake is known to cause fluorosis, osteosarcoma, and arthritis. Alzheimer's disease is also believed to be associated with uptake of toxic aluminum fluoride through drinking water. Because heretofore there has been no simple and inexpensive way of detecting fluoride in water, a person may consume an undetermined amount of fluoride every day, which may enhance the risk for severe health conditions during the latter stages of life.
Water fluoridation, which involves the addition of F− to tap water, has been a common practice in the United States since the 1950s. Tap water fluoridation, however, has recently come under increased scrutiny. P. Connet, Fluoride, 2007, 40, 155-158; (b) R. J. Carton, Fluoride, 2006, 39, 163-172. The EPA recommends a minimum F− concentration in drinking water of 0.7 ppm, a level sufficient to provide benefits to dental and skeletal health. However, concentrations over 2 ppm are considered a risk to human health, and higher doses are known to cause the debilitating conditions listed above. Accordingly, proper maintenance of such a narrow F− concentration tolerance demands highly sensitive and selective detection techniques.
Generally, the sensing and detection of anions using non-covalent interaction, such as anion-π interaction, electrostatic and hydrogen bonding interaction, is an emerging area of current research. For example, it is known that a number of hydrogen bond (H-bond) donating receptors are able to bind F− anions via H-bond formation. Previous studies have described F− sensors using compounds based on urea, thiourea, amide, sulfonamide, pyrole, and indole, among others, which utilize this technique. Colorimetric sensing of fluoride is particularly desirable, and has previously been studied as a possible detection method for the nerve gas Sarin (GB) (isopropyl methylphosphonofluoridate), which loses a fluoride anion during hydrolysis. R. M. Black, J. M. Harrison, R. W. Read Arch Toxicol 1999, 73, 123-126.
Because of the non-chromogenic nature of most Y—H . . . X— H-bonds, however, hydrogen bond donating receptors either rely on adjacent chromophore units or deprotonation of acidic protons followed by electron delocalization to display colorimetric response. As a result, the fluoride detection mechanism is usually not reversible, and prevents the compounds from being easily reused. Furthermore, compounds using this mechanism rarely discriminate between strongly basic anions (e.g., F−, acetate anion (AcO−), and H2PO4−), and often show poor selectivity and sensitivity for the F− anion as a result.
Comparatively, anion-π interaction mechanisms have received less research attention.
Maeda has reported a metal complex that showed a high association constant, Ka for F− dissolved in dichloromethane (>3×105 M−1). This result was ascribed not only to the acidity of the NH peripheral group, but also to the anion-π interaction between the F− and the closest electron deficient fluorinated phenyl ring. Mascal has proposed novel cylindrophane-type receptors based on π-electron deficient rings, which demonstrate a high level of selectivity for F−, both in the gas phase and in aqueous solvent model. See M. Mascal Angew. Chem. Int. Ed. 2006, 45, 2890-2893.
1,4,5,8-naphthalenediimides (NDIs) have attracted much attention due to their tendency to form n-type (over p-type) semiconductor materials, which are often used in applications such as electron donor-acceptor dyes and molecular machines. S. V. Bhosale, C. H. Jani, S. J. Langford Chem. Soc. Rev., 2008, 37, 331-342. (b) H. E. Katz, A. J. Lovinger, C. Kloc, T. Siegrist, W. Li, Y.-Y. Lin, A. Dodabalapur, Nature, 2000, 404, 478-481. (c) N. Sakai, R. S. K. Kishore, S. Matile Org. Biomol. Chem., 2008, 6, 3970-3976. However, the ability of NDI to interact with anions is relatively less explored.
Recently, Matile and coworkers have reported a synthetic ion channel based on NDI rods as transmembrane anion-π-slides. V. Gorteau, G. Bollot, J. Mareda, A. P.-Velasco, S. Matile J. Am. Chem. Soc. 2006, 128, 14788-14789 (b) J. Mareda, S. Matile Chem. Eur. J. 2009, 15, 28-37 (c) V. Gorteau, G. Bollot, J. Mareda, Stefan Matile Org. Biomol. Chem., 2007, 5, 3000-3012. A core-substituted NDI based fluoride sensor has also been reported, in this case having a two-stage deprotonation process leading to a colorimetric response. S. V. Bhosale, S. V. Bhosale, M. B. Kalyankar, S. J. Langford Org. Lett. 2009, 11, 5418-5421.
Iverson and others demonstrated that charge transfer and π-π-stacking interactions, which occur between a colorless NDI unit and electron rich aromatic rings, produce donor-acceptor charge transfer complexes having a colorimetric response. However, no research has previously investigated the effect of anion-π interaction between NDI units and F− anions.
A need persists in the art for a method of fluoride anion detection that has good sensitivity, shows high selectivity for fluoride, and is economical for widespread use.